1. Field of the Invention
The present invention relates to a semiconductor device. In particular, the present invention relates to an LDMOS (Lateral Diffused Metal Oxide Semiconductor) transistor.
2. Description of Related Art
As a high breakdown voltage transistor, an LDMOS transistor is known. In the LDMOS transistor, a thick insulating film, which differs from a gate insulating film, is formed in a semiconductor substrate between a gate electrode and a drain diffusion region. Such an insulator structure formed between a gate and a drain is hereafter referred to as “field drain section”. For example, according to a technique described in a patent literature 1 (Japanese patent publication number JP2008-535235A: International publication number WO2006/103634), the field drain section is formed by using the same process as the STI (Shallow Trench Isolation) structure serving as an element isolation structure. A drain-to-substrate breakdown voltage (BVds)) is improved by the foregoing field drain section.
The inventor has now discovered the following facts. According to the consideration of the inventor, it was found that there is room to further research in order to improve the breakdown voltage of the LDMOS transistor. For example, impact ionization is apt to be generated at a boundary between the field drain section and the semiconductor substrate. The impact ionization implies a phenomenon in which many electron-hole pairs are generated by collisions between crystal lattices and electrons accelerated by an electric field. The electron-hole pairs, which are generated by the foregoing impact ionization, disturb the improvement of the drain-to-substrate breakdown voltage BVds. That is, the impact ionization causes the breakdown voltage improvement to be saturated. The breakdown voltage of the LDMOS transistor is desired to be further improved.